Which is better a combi or system boiler?

boiler, which is better, solar pv, solar thermal

One of the trends of the domestic heating industry in the last twenty or so years has been the switch from system boiler to combi boiler. A combi boiler provides hot water straight from the boiler so that there is no need for hot water storage. The main advantage of this is all the space it frees up in the airing cupboard so you’ve got lots of space to put all the rubbish you can’t agree with your spouse should be thrown out!

In addition to this, much emphasis was put on the energy efficiency of combi boilers, especially as when they first arrived on the scene, storage tanks were about as well insulated as a Geordie on a night out. Storing hot water was inefficient, took up lots of space and frankly with the scalding possibility from uninsulated copper heating-dangerous.

However, times have changed and I think its about time the balance was redressed. Hot water cylinders, whether vented or unvented are not the inflexible dinosaurs of old. If anything, they form a key part of a energy efficient building and careful thought should be given before discarding them.

First thing that needs to be said is that a combi provides reliable hot water only on small properties. There may be some exception to this but its generally true. A decent combi can probably provide ok hot water pressure to two showers at a push but beyond that, forget it! A hot water tank on the other hand, provided that the incoming pressure is good and distribution pipes have been sized accurately etc. can give hot water to pretty much as many outlets as you like. This is potentially an issue on fairly large houses and any kind of commercial premises where multiple outlets are required simultaneously.

Secondly, hot water tanks are not the uninsulated monstrosities that they used to be. In old airing cupboards you will see hot water tanks with loads of bare copper showing and a thin old jacket sellotaped on (on occasion I have seen actual coats…for humans!!!) and its like a sauna in there. A modern hot water thank though is so much better. Its foam insulation integral to the tank (ie it can’t just fall off). You could literally cuddle one of these things and have no idea if there is hot or cold water in it.

Where a hot water tank really comes into play though is its potential for optimising renewable energy sources, particularly solar thermal and solar pv. Solar thermal works by thermally absorbing heat from the sun onto rooftop collectors and passing that heat into stored water. It is possible to use the heat for space heating but this is generally quite a complicated process and not really suitable for domestic. There are a couple of combi boilers on the market that will take pre-heated water but generally the best way to use this heat is through a storage tank.

A hot water tank is also one of the best ways to make full use of solar pv. The nature of solar pv is that generation goes up and down with the solar irradiation. It is often the case that when you are generating lots of solar electricity you don’t actually have a use for it at that moment in time. In can be exported to the grid but it is much better to use it if you can.

A product known as a solar hot water optimiser can come to the rescue. These work by constantly registering the solar pv generation and how much of this electricity is being utilised. Should an excess be detected then the immersion heater in the hot water cylinder is switched on to the relevant power. Apart from during the depths of winter, this will comfortably give a full tank of hot water daily. Obviously, this amounts to a serious saving in gas or oil over the course of a year. Its a no-brainer if you have both solar pv and a hot water tank.

One final word on the space issue. Obviously a tank does take up space, no two ways about it. However, there are options. There are slim line versions, there are even horizontal versions. They don’t have to go in the cupboard, they can go in the loft. If you’re getting a new one, have a good chat with your heating engineer and see what can be done.


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